[wyum]

Weird - I just experienced one of these as I was waking up about 20 minutes ago!

I say "weird" b/c it's not a regular thing, but I have experienced this a few other times, along with similar sudden full-body "spasms" while drifting to sleep. It's not usually alarming, just a strange feeling.

[taneq]

The full body spasms are normal, they’re called “hypnic jerks” (https://en.m.wikipedia.org/wiki/Hypnic_jerk)

I’m guessing the exploding head thing is related but involves auditory nerves rather than the semicircular canals?

[rzzzt]

One hypothesis for the sudden contraction of muscles is that your brain is losing feeling of the bed supporting your body when going to sleep: https://en.wikipedia.org/wiki/Hypnic_jerk

[saghm]

When this happens to me, it always feels like I'm about to fall forward. The best I can describe it is as the feeling that I just took a step forward but instead of my foot finding the ground, the surface I was walking on ended (as if I stepped off a ledge or something).

[rzzzt]

For me it's the opposite: it feels like someone pushing me out of a tree swing or a chair that has no backrest (I'm sleeping on my side).